An alternative way to solve sentence-improvement questions like this is to insert each option in place of the underlined words and test whether the full sentence reads naturally.
Only inserting "so severe" produces a grammatically standard sentence with the correct cause-and-effect structure.
Hence, the correct answer is so severe.
Another way to work through this preposition-correction question is to test each option against the standard collocations used for modes of travel in English.
Matching the sentence against the standard travel-mode collocation confirms that "by" is the preposition required here.
Hence, the correct answer is by.